Ethernet has become a successful media interface for local area networks (LANs) and has seen an exponential growth in achievable data rates over the past decade. In 1999, the IEEE 802.3ab task force specified the first Gigabit Ethernet over copper standard, 1000BASE-T, which allows for 1 Gb/s communications over four wire pairs of Category 5 unshielded twisted pair (UTP) copper cabling for distances up to 100 m. However, due to the channel characteristics of Category 5 cabling and due to the fact that full duplex transmission is employed on each wire pair, a 1000BASE-T receiver corresponding to one wire pair must cope with a number of impairments, including intersymbol interference (ISI) caused by wire attenuation, echo from its own transmitter, near end crosstalk (NEXT) from the adjacent three local transmitters, far end crosstalk (FEXT) from the remote transmitters of the three adjacent wire pairs, and noise from other sources.
1000BASE-T Gigabit Ethernet employs 4-dimensional trellis-coded modulation (4D-TCM) to improve the noise margin. For a more detailed discussion of 4D-TCM encoding, see, for example, G. Ungerboeck, “Trellis-Coded Modulation With Redundant Signal Sets, Parts I and II,” IEEE Commun. Mag., vol. 25, pp. 5-21 (February 1987). Whereas FEXT does not need to be cancelled in a 1000BASE-T implementation, the noise from ISI, echo and NEXT impairments must be removed to achieve a target bit error rate of at least 10−10. In a digital signal processor (DSP) based VLSI implementation, the number of filter taps needed for the respective cancellers is very high leading to a VLSI implementation with significant power consumption.
Therefore, a Gigabit Ethernet scheme employing four pairs of Category 6 UTP cabling was proposed in K. Azadet et al., “A Gigabit Transceiver Chipset for UTP CAT-6 Cables in Digital CMOS Technology,” IEEE Int. Solid-States Circuits Conf. (ISSCC) Dig. Tech. Papers, San Francisco, Calif., 306-307 (February 2000), that leads to a transceiver implementation with significantly lower hardware complexity. Category 6 UTP cabling has significantly better channel characteristics than Category 5 UTP cabling, as it has a bandwidth of 250 MHz (Category 5: 100 MHz) and as it suffers less from NEXT.
The transmission scheme for Category 6 Gigabit Ethernet is shown in FIG. 1. Each of the four wire pairs transmits data at a rate of 500 Mb/s only in one direction, and two wire pairs are used to achieve an overall data rate of 1 Gb/s for distances up to 100 m. Thus, there is no echo, and there are only two NEXT sources (for 1000BASE-T there are three NEXT sources) and 1 FEXT source (for 1000BASE-T there are 3 FEXT sources) on a single wire pair. For Category 6 UTP cabling, the impairments of NEXT and FEXT are so weak that they do not need to be cancelled to achieve a target bit error rate of at least 10−10. Also, the postcursor ISI after feedforward equalization is significantly shorter for Category 6 than for Category 5 cabling. Therefore, only ISI must be cancelled in Category 6 Gigabit Ethernet using adaptive equalization, leading to a VLSI implementation with significantly lower hardware complexity and power consumption than the 1000BASE-T standard.
As there is significant postcursor ISI in Category 6 Gigabit Ethernet, 4D-TCM must be employed as well to increase the noise margin. To benefit from the potential coding gain of this coding scheme, joint equalization and trellis decoding must be performed. However, an optimum maximum likelihood sequence estimation (MLSE) detector would be far too complex, and the conventional suboptimum reduced-state sequence estimation (RSSE) algorithm cannot be applied, as the number of channels used for data transmission is smaller than the number of dimensions of the 4D-TCM code. In other words, under a 4D-TCM encoding scheme, two symbols corresponding to two dimensions of the four dimensional code are transmitted successively over the same wire pair. A need therefore exists for a joint postcursor equalizer and trellis decoder for such multidimensional codes, such as the trellis-coded Category 6 Gigabit Ethernet scheme. A further need exists for an RSSE scheme that cancels the intrasymbol interference caused by other symbol components within the same multidimensional code symbol, as well as the intersymbol interference caused by previously transmitted multidimensional code symbols.